Method for producing polymethacrylimide foams

ABSTRACT

The invention relates to an improved method for producing foamed material, especially poly(meth)acrylimide foams, which are foamed from polymer plates produced according to the casting method. The two-step method consists of a pre-heating step and at least one foaming step. The product obtained has a significantly smaller compressive strain, measured according to DIN 53425(ASMD 621), than prior art products.

FIELD OF THE INVENTION

The invention relates to an improved method for the preparation offoams, in particular poly(meth)-acrylimide foams, which are foamed frompolymer sheets produced by the casting method. The two-stage methodconsists of a preheating step and one or more foaming steps.

DISCUSSION OF BACKGROUND

Polymethacrylimide foams have long been known and, owing to theirexcellent mechanical properties and their low weight, are widely used,in particular in the production of multilayer materials, laminates,composites or foamed composites. Prepregs comprising polymethacrylimidecore materials are frequently bonded here.

For example, they are used in aircraft construction, in shipbuilding aswell as in automotive construction. For many of these numerousapplications, they have to meet technical requirements laid down instatutory provisions and a number of other regulations.

The present invention relates to the area of the polymer blocks producedby the casting method and polymethacrylimide foams prepared therefrom.Here, the monomers methacrylic acid and methacrylonitrile are introducedbetween two plane-parallel plates —generally glass plates. After thepolymerization, the polymer sheets obtained are foamed in a further,separate method step.

The method relevant in production technology is based on foaming in ahot-air oven, which is to be referred to below as the hot-air method.The polymer sheets are introduced suspended in a forced-circulationoven, transported through said oven by a self-sustaining traction systemand discharged at the end as foam sheets. The distance covered by thesheets in the oven is referred to below as L. The foaming time is thusdefined by the length L of the oven and the constant travelling velocityV of the transport system in the total oven. The oven throughput dependsnot only on its length L and the travelling velocity V of the transportsystem but also on the time interval t and hence also the geometricspacing a of the sheets with which the latter are introduced into theoven. Since the sheets are greatly distorted during the foaming method,the spacing a must be larger than b/π so that the sheets cannot touchone another during the foaming and thus become damaged. b is defined asthe length of the side from which the sheet is suspended and which thesheet has when it has been foamed. The content of this publication islimited to the method step comprising foaming.

DE 3 630 960 describes a further method for the foaming of theabovementioned copolymer sheets from methacrylic acid andmethacrylonitrile. Here, the sheets are foamed with the aid of amicrowave field, and this is therefore referred to below as themicrowave method. It must be ensured here that the sheet to be foamed orat least its surface must be heated beforehand up to or above thesoftening point of the material. Since of course the foaming of thematerial softened by the external heating also begins under theseconditions, the foaming method alone cannot be controlled by theinfluence of a microwave field but also must be controlled from theoutside by accompanying heating. Thus, a microwave field is coupled withthe usual one-stage hot-air method in order to accelerate the foaming.However, the microwave method has proved too complicated and thereforenot relevant in practice and is not used today.

WO 90/2621 describes a foam obtained from methacrylic acid andmethacrylonitrile, acrylamide as a comonomer preventing prematureformation of precipitates during the polymerization. The foam formed isvery uniform and the product has no internal stresses.

DE 197 17 483 describes a method for the preparation ofpolymethacrylimide foams to which 1–5% by weight, based on the monomermixture, of MgO are added. Foams having substantially improvedthermomechanical properties are obtained.

DE 196 06 530 describes the addition of a flameproofing agent by meansof polymethacrylimide foams.

OBJECT

In order to make ROHACELL more attractive for existing applications, itis necessary to optimize its material properties. Heat of reactionevolved during the foaming leads to a temperature gradient in the foamedsheet and therefore also to a location-dependent density in the sheet.As a result of this, the mechanical characteristics of a foam sheetlikewise depend on the sampling location, since the density is known tohave a considerable effect on mechanical properties, such as, forexample, compressive strength or creep behaviour. The heat of reactionevolved can lead to cracking and hence to the destruction of thematerial in the production of low densities. It has now been found thatthe abovementioned disadvantages can be avoided by the method found. Forthis purpose, a more efficient preparation is to be ensured by anassociated increase in the throughput.

ACHIEVEMENT

Surprisingly, the object described above can be achieved by dividing thehot-air method into two separate hot-air processes. Instead of twohot-air processes, it is also possible to combine three or moreprocesses. In the first hot-air process, the sheet to be foamed ispreheated in a hot-air oven below the actual foaming temperature of thematerial. The linear regression of the temperature increase as afunction of time gives a mean linear heating rate of 0.001–10 K/min,preferably 0.01–5 K/min and particularly preferably 0.1–1 K/min.

The linear regression of the temperature increase is also referred to asthe temperature ramp. The hot sheet is transported from the preheatingoven into the actual foaming hot-air oven. The foaming hot-air oven hasthe temperature required for foaming, which is above the preheatingtemperature. The foaming hot-air oven can also consist of a second ovenpart of the preheating oven. The temperature profile to which the sheetis subjected during the foaming is represented by the grey line inFIG. 1. The high viscosity in the low temperature range of thepreheating inevitably results in a supersaturated solution of theblowing gas in the polymer. The evolved heat of reaction, which isusually troublesome during the foaming, is uniformly distributed in thepolymer sheet on preheating. Only when the material is heated to thefoaming temperature does phase separation of polymer matrix and blowingagent occur and lead to expansion of the polymer sheet.

BRIEF DESCRIPTION OF THE FIGURE

The preheating can be carried out here in the form of a temperature rampor of a constant preheating temperature. FIG. 1 shows the differencebetween the method to date (black line, one-stage hot-air method) andthe novel method (broken line, two-stage hot-air method) by way ofexample for the case of a constant preheating temperature.

Advantages of the method according to the invention:

In the case of certain formulations, PMI foams have poor creep behaviourif they are foamed in a one-stage method step. This makes processing ofsuch foams as core material possible only to a limited extent. With theaid of the two-stage hot-air method, the compression according to DIN53425 (ASTMD621) can be reduced to 1/10.

Furthermore, cracking can occur in foam slabs in the case of certainformulations when the one-stage hot-air method is used for theproduction of low densities, which leads to waste. Foam slabs which havecracks owing to imperfect foaming and therefore cannot be used forapplications are to be regarded as waste here. Cracks must not occur.Thus, for example, 40% waste means that 40 out of 100 foam slabsproduced have to be removed and disposed of owing to imperfect foamingand/or cracking. With the aid of the two-stage hot-air method, the wastecan be more than halved.

Because the actual foaming time can be reduced by upstream preheating,the travelling velocity V of the transport system in the oven can beincreased in the case of a two-stage hot-air method, which causes thethroughput to increase. FIG. 1 shows, by way of example, this reductionin the foaming time by the preheating of the polymers, withoutrestricting this effect to the parameters shown there: the foaming timeis reduced to ⅔ of the original foaming time in this example.

If the uniformly preheated polymer sheet is further heated to thefoaming temperature, no temperature gradient is caused in the sheet byan exothermic reaction and furthermore the temperature gradient due tothe temperature jump to the foaming temperature is itself smaller. Thelarger this temperature jump which the polymer sheet experiences onentering the foaming process, the greater is the temperature gradientcaused thereby and produced in the sheet.

It is obvious that stress differences and blowing agent pressuredifferences occur in the material, firstly owing to the thermalexpansion and secondly owing to the staggered start of foaming, which islocation-dependent because of the temperature gradient. In the exampleshown in FIG. 1, the temperature jump experienced by the polymer sheeton entering the foaming process is 175 K for the case of the one-stagehot-air method (black line) and only 40 K for the case of the two-stagehot-air method (grey line).

By means of a suitable (temperature ramp), it is also possible entirelyto avoid a temperature jump. This finally has a major consequence forthe homogeneity of the foam sheet: the initially described distortion ofthe sheets can be suppressed so that the condition a>b/π no longer needbe maintained. This shortens the cycle time t introduced at the outsetand, owing to the increase in the throughput, also has an ecologicalbenefit in addition to the increased net product with the same ovendesign.

EXAMPLES Comparative Example 1

330 g of isopropanol and 100 g of formamide were added as blowing agentto a mixture of 5 700 g of methacrylic acid, 4 380 g ofmethacrylonitrile and 31 g of allyl methacrylate. Furthermore, 4 g oftert-butyl perpivalate, 3.2 g of tert-butyl per-2-ethylhexanoate, 10 gof tert-butyl perbenzoate, 10.3 g of cumyl perneodecanoate, 22 g ofmagnesium oxide, 15 g of blowing agent (PAT 1037) and 0.07 g ofhydroquinone were added to the mixture.

This mixture was polymerized for 68 h at 40° C. and in a chamber formedfrom two glass plates measuring 50×50 cm and having an 18.5 mm thickedge seal. The polymer was then subjected to a heating programme rangingfrom 32° C. to 115° C. for 32 h for the final polymerization.

The subsequent foaming in the hot-air method was carried out for 2 h 25min at 205° C., considerable distortion of the sheet being observableduring the foaming. In the incompletely foamed state, the sheet curvedat one point to such an extent that the two opposite sides which areperpendicular to the suspension side touched at one point. The foam thusobtained had a density of 235 kg/m³. The compression according to DIN53425 (ASTM D621) was more than 18% at 180° C. and a load of 0.35 MPaafter 2 h.

Example 1

The procedure was as described in comparative example 1. However, thehot-air method used was in two stages: preheating was effected for 2 hat 140° C. and then foaming for 2 h 75 min at 205° C. Only negligibledistortion of the foamed sheet was observed. The foam thus obtained hada density of 238 kg/m³. The compression according to DIN 53425 (ASTMD621) was 12.7% at 180° C. and a load of 0.35 MPa after 2 h.

Example 2

The procedure was as described in comparative example 1. However, thehot-air method used was in two stages: preheating was effected for 2 hat 150° C. and then foaming for 2 h 25 min at 210° C. Only negligibledistortion was observed, which was less than in Example 1.

The foam thus obtained had a density of 203 kg/m³. The compressionaccording to DIN 53425 (ASTM D621) was 4.6% at 180° C. and a load of0.35 MPa after 2 h.

Example 3

The procedure was as described in comparative example 1. However, thehot-air method used was in two stages: preheating was effected for 2 hat 160° C. and then foaming for 2 h 25 min at 215° C. Only negligibledistortion was observed, which was less than in example 2. The foam thusobtained had a density of 208 kg/m³. The compression according to DIN53425 (ASTM D621) was 2.9% at 180° C. and a load of 0.35 MPa after 2 h.

Example 4

The procedure was as described in comparative example 1. However, thehot-air method used was in two stages: preheating was effected for 2 hat 160° C. and then foaming for 2 h 25 min at 220° C. Only negligibledistortion was observed, which was similar to that in example 3. Thefoam thus obtained had a density of 168 kg/m³. The compression accordingto DIN 53425 (ASTM D621) was 1.3% at 180° C. and a load of 0.35 MPaafter 2 h.

Example 5

The procedure was as described in comparative example 1. However, thehot-air method used was in two stages: preheating was effected for 2 hat 170° C. and then foaming for 2 h 25 min at 215° C. No distortion wasobserved. The foam thus obtained had a density of 199 kg/m³. Thecompression according to DIN 53425 (ASTM D621) was 3.5% at 180° C. and aload of 0.35 MPa after 2 h.

Example 6

The procedure was as described in comparative example 1. However, thehot-air method used was in two stages: preheating was effected for 1 h25 min at 180° C. and then foaming for 2 h 25 min at 210° C. Nodistortion was observed. The foam thus obtained had a density of 218kg/m³. The compression according to DIN 53425 (ASTM D621) was 1.6% at180° C. and a load of 0.35 MPa after 2 h.

Comparative example 1 and examples 1 to 6 clearly show that the creepbehaviour is improved by the preheating. In spite of lower densities, alower compression is observed under identical measuring conditions. Onthe other hand, it is known to a person skilled in the art that areduction in the density of a rigid foam results in a deterioration inits mechanical properties, i.e. its creep modulus becomes smaller andhence the compression greater under identical measuring conditions.

Comparative Example 2

42 kg of isopropanol and 47 kg of formamide were added as blowing agentto a mixture of 610 kg of methacrylic acid and 390 kg ofmethacrylonitrile. Furthermore, 0.4 kg of tert-butyl perpivalate, 0.4 kgof tert-butyl per-2-ethylhexanoate, 0.7 kg of tert-butyl perbenzoate,1.03 kg of cumyl perneodecanoate, 2.2 kg of zinc oxide, 1.5 kg ofblowing agent (PAT 1037) and 0.075 kg of hydroquinone were added to themixture.

This mixture was polymerized for 116 h at 33° C. in chambers which wereformed from two glass plates measuring 100×200 cm and having a 30 mmthick edge seal. The polymer was then subjected to a heating programmeranging from 35° C. to 130° C. for 40 h for the final polymerization.

The subsequent foaming in the hot-air method was effected for 2 h 30 minat 200° C., considerable distortion of the sheets being observableduring the foaming. The foam thus obtained had a density of 31 kg/m³.However, 40% of the foam thus prepared had to be discarded as waste,owing to cracking.

Example 7

The procedure was as described in comparative example 2. However, thehot-air method used was in two stages: preheating was effected for 1.5 hat 160° C. and then foaming for 2 min 30 min at 205° C. No distortion ofthe sheets was observed during the foaming. The foam thus obtained had adensity of 32 kg/m³. Cracking and the associated material loss due towaste could be reduced to 5%.

1. A method for production of polymethylacrylimide foamed materials inthe form of blocks or plates, comprising: copolymerizing methacrylicacid and methacrylonitrile and optionally a copolymerizable monomer andadditive in the presence of a radical-forming initiator, therebyobtaining a copolymer; postpolymerizing and cyclizing said copolymer tosaid polymethylacrylimide, and transforming said polymethylacrylimide toa foamed material, in a two-stage process step, wherein a first stage ofsaid two-stage process step comprises preheating of thepolymethylacrylimide to be foamed in a first hot-air oven or hot airoven section at a heating rate used to raise the temperature of between0.001 K/min and 10 K/min, while foaming of the preheatedpolymethylacrylimide takes place in a second stage of said two-stageprocess, in a second hot-air oven or hot-air ovens or hot air ovensection.
 2. The method according to claim 1, in which the two hot-airovens or hot-air oven sections used for the two-stage process havedifferent temperatures.
 3. The method according to claim 1, in which thetemperature of the hot-air oven used for preheating is lower, whilebeing constant in time, than that of the hot-air oven or hot-air ovensection used for foaming.
 4. The method according to claim 1, in whichthe temperature of the hot-air oven used for preheating is lower, whilerising over the course of time, than that of the hot-air oven or hot-airoven section used for foaming, and wherein the temperature in thehot-air oven used for preheating can once again be equal, at the end ofthe heating cycle, to the temperature in the hot-air oven or hot-airoven section used for foaming.
 5. The method according to claim 1, inwhich the heating rate used to raise the temperature is between 0.01K/min and 5 K/min.
 6. The method according to claim 1, in which theheating rate used to raise the temperature is between 0.1 K/min and 1K/min.
 7. The method according to claim 6, in which different heatingrates in combination with one another can be used for the average lineartemperature rise.
 8. The method according to claim 6, in which the finaltemperature of the temperature rise can be higher than the temperaturethat is needed for foaming and that exists in the hot-air oven used forfoaming.
 9. The method according to claim 1, wherein saidcopolymerizable monomer is present.
 10. The method according to claim 1,wherein said additive is present.
 11. The method according to claim 1,wherein said first stage of said two-stage process step is performed inat least one hot air oven.
 12. The method according to claim 1, whereinsaid first stage of said two-stage process step is performed in at leastone hot air oven section.
 13. The method according to claim 1, whereinsaid second stage of said two-stage process step is performed in atleast one hot air oven.
 14. The method according to claim 1, whereinsaid second stage of said two-stage process step is performed in atleast one hot air oven section.